When the bit rate of moving picture information to be transmitted exceeds the capacity of a transmission system, pictures (or frames) have to be thinned out. That is, frames have to be skipped. This necessity arises also when the amount of processing for real-time encoding exceeds the capacity of an encoding apparatus.
Also, additional information such as character information is sometimes transmitted together with moving picture information. For example, character information for a news bulletin is sometimes transmitted while information for a TV program is being transmitted. Such additional information is basically transmitted in association with the corresponding pictures. In the example illustrated in FIG. 1, additional information A is added respectively to consecutive pictures P1 through P4.
When additional information is transmitted together with moving picture information, since the additional information is transmitted in association with the corresponding pictures as described above, it is not preferable to thin out a picture with additional information. Thus, when a picture with additional information is to be thinned out, a dummy picture has to be inserted in place of the thinned out picture. In the example illustrated in FIG. 1, picture P3 is thinned out and a dummy picture Pd is inserted in place of picture P3. Additional information A is added respectively to pictures P1, P2, Pd, and P4 and transmitted. In addition, the amount of information of a dummy picture is desirably as small as possible. Further, the amount of processing that has to be performed by an encoding apparatus to generate a dummy picture is also desirably as small as possible.
Newer moving picture encoding apparatuses are usually equipped with a function of compressing information by referring to a different picture in order to encode data of each picture. This function includes a process of generating encoded data from motion information and an error between an encoding target picture and a reference picture. Accordingly, when a target picture refers to another picture, the encoded data of the target picture includes information used for identifying the reference picture. Thus, when a target picture referring to another picture is replaced with a dummy picture, the encoded data of that dummy picture also needs to include information for identifying the reference picture.
In conventional moving picture encoding methods such as MPEG2, the degree of freedom for reference pictures is low (only a small number of pictures can be referred to), and accordingly the relationship between an encoding target picture and a reference picture is simple. Thus, in a configuration that dummy data is prepared in advance for thinned-out pictures, the number of dummy pictures is small.
However, in H.264, which provides an encoding efficiency higher than MPEG2, the degree of freedom for reference pictures is high (a greater number of pictures can be referred to). Also, H.264 permits each of the blocks constituting a picture to refer to one or a plurality of different pictures. Therefore, a greater number of dummy pictures are required in a configuration that prepares dummy pictures in advance for thinned-out pictures. Specifically, as many pieces of different dummy data have to be prepared as referenceable pictures. Accordingly, when dummy pictures are prepared in advance and stored in a memory area, the memory area becomes large. In the example illustrated in FIG. 2, dummy pictures d1, d2, d3, . . . are stored in a memory area, and dummy picture d2 is read out and used.
As a related art, Patent Document 1 discloses an image encoded-data generating apparatus that thins out frames according to the content of an image. This apparatus has a function of dividing encoded data in frames into encoded data of image areas and encoded data of caption areas, and merging the encoded data of image areas with the encoded data of caption areas after thinning out the encoded data of image areas.
As another related art, Patent Document 2 discloses a method for detecting an area not having information to be encoded in order to reduce the total amount of information to be encoded.    Patent Document 1: Japanese Laid-open Patent Publication No. 2000-32448    Patent Document 2: Japanese Laid-open Patent Publication No. 2002-281508